1. Technical Field of the Invention
This invention generally relates to an automotive air conditioning system, and more particularly, to an automotive air conditioning system having a heat pump type refrigerant circuit.
2. Description of the Prior Art
Automotive air conditioning systems having a heat pump type refrigerant circuit are well known in the prior art.
With reference to FIG. 1, one conventional automotive air conditioning system 100' comprises a heat pump type refrigerant circuit 10' which includes a compressor, such as a variable displacement type motor driven compressor 11, a first internal heat exchanger 12, a second internal heat exchanger 13, a receiver 14 and an external heat exchanger 15, which are connected to one another by pipe members 20 in a manner described in detail below.
An outlet port 11b of compressor 11 is linked to an inlet port 13a of second internal heat exchanger 13 via a first electromagnetic valve 161. An outlet port 13b of second internal heat exchanger 13 is linked to an inlet port 14a of receiver 14 via a first check valve 171. An inlet port 11a of compressor 11 is linked to an outlet port 12b of first internal heat exchanger 12. A first port 15a of external heat exchanger 15 is linked to pipe member 20 at a position which is located between inlet port 11a of compressor 11 and outlet port 12b of first internal heat exchanger 12 via a second electromagnetic valve 162. First port 15a of external heat exchanger 15 is further linked to pipe member 20 at a position which is located between outlet port 11b of compressor 11 and first electromagnetic valve 161 via a third electromagnetic valve 163.
A second port 15b of external heat exchanger 15 is linked to inlet port 14a of receiver 14 via a second check valve 172. An outlet port 14b of receiver 14 is linked to an inlet port 12a of first internal heat exchanger 12 via a fourth electromagnetic valve 164 and a second valve device 502 of a later-mentioned second external pressure equalizing type thermal-sensitive expansion valve mechanism 52. Outlet port 14b of receiver 14 is further linked to second port 15b of external heat exchanger 15 via a first valve device 501 of a later-mentioned first external pressure equalizing type thermal-sensitive expansion valve mechanism 51.
The automotive air conditioning system 100' further comprises a duct 110 within which first internal heat exchanger 12, second external heat exchanger 13 and a blower 120 are disposed. An air intake port 111 is formed at one end of duct 110. Blower 120 is located at a position adjacent to air intake port 111, and moves the air from air intake port 111 to the opposite end of duct 110. A first damper 130 is associated with air intake port 111, and pivotally moves so that fresh outside air or recirculated passenger compartment air can be selectively taken into duct 110 through air intake port 111.
First internal heat exchanger 12 is disposed within duct 110 at a position downstream of blower 120. Second internal heat exchanger 13 is located at a position downstream to first internal heat exchanger 12. Second internal heat exchanger 13 occupies about one half of the cross sectional area of duct 110. A second damper 140 is associated with second internal heat exchanger 13 at a position immediately upstream with respect to heat exchanger 13, and pivotally moves so that the flow of air passing through second internal heat exchanger 13 is continuously regulated.
The other end of duct 110 opposite to air intake port 111 branches into first and second auxiliary ducts 110a and 110b. First auxiliary duct 110a conducts the air to an automobile front windshield (not shown). The second auxiliary duct 110b conducts the air to a front lower portion of the automobile passenger compartment.
A first thermo-sensor 30 is associated with first internal heat exchanger 12 at a position downstream with respect to heat exchanger 12, and senses temperature T.sub.eva of the air flowing out from heat exchanger 12. The sensed temperature T.sub.eva is converted to an electric signal S.sub.eva at first thermo-sensor 30, and electric signal S.sub.eva is sent to a control unit (not shown) to be processed therein. A second thermo-sensor 40 is associated with second internal heat exchanger 13 at a position downstream with respect to heat exchanger 13, and senses temperature T.sub.mix of the air flowing through heat exchanger 13. The sensed temperature T.sub.mix is converted to an electric signal S.sub.mix at second thermo-sensor 40, and electric signal S.sub.mix is sent to the control unit to be processed therein.
The heat pump type refrigeration circuit 10' of the automotive air conditioning system 100' is provided with first and second external pressure equalizing type expansion valve mechanisms 51 and 52. First external pressure equalizing type expansion valve mechanism 51 includes a first valve device 501, a first pressure equalizing pipe 51a which functions as a pressure sensing device, and a first thermo-sensing tube 51b, which functions as a thermo-sensing device. Similarly, second external pressure equalizing type expansion valve mechanism 52 includes a second valve device 502, a second pressure equalizing pipe 52a, which functions as a pressure sensing device, and a second thermo-sensing tube 52b, which functions as a thermo-sensing device.
With reference to FIG. 2 in conjunction with FIG. 1, first valve device 501 is disposed in pipe member 20 at a position between second port 15b of external heat exchanger 15 and outlet port 14b of receiver 14. One end of first pressure equalizing pipe 51a is hermetically inserted into an inner space of tube member 20 between first port 15a of external heat exchanger 15 and second electromagnetic valve 162. First thermo-sensing tube 51b is fixedly attached to an external surface of tube member 20 between second electromagnetic valve 162 and inlet port 11a of compressor 11.
First valve device 501 includes a casing 511, which defines a pressure sensitive operating chamber 512, and a diaphragm 513 disposed within and dividing chamber 512 into a first section 512a and a second section 512b. An operating rod 514 is disposed within second section 512b of chamber 512, and one end thereof slidably penetrates through casing 511 and is located in a communicating chamber 515, which is formed in casing 511. The communicating chamber 515 is in fluid communication with pipe member 20. Valve element 514a is integrally formed at one end of rod 514, and is resiliently supported by a coil spring 516. Coil spring 516 is disposed between valve element 514a and a supporting member 517, which is fixedly secured to an inner surface of communicating chamber 515. Operating rod 514 is continuously urged upwardly (with respect to FIG. 2) by the restoring force of coil spring 516. As a result, the end of operating rod 514 is continuously in contact with a central portion of diaphragm 513. A valve seat 518 is formed in communicating chamber 515 facing valve element 514a.
First thermo-sensing tube 51b is linked to the first section 512a of pressure sensitive operating chamber 512 through pipe member 51c. First pressure equalizing pipe 51a is connected to the second section 512b of pressure sensitive operating chamber 512. First thermo-sensing tube 51b, pipe member 51c and first section 512a of pressure sensitive operating chamber 512 are charged with refrigerant. The refrigerant charge is maintained at the saturated vapor pressure, which varies in accordance with the Mollier diagram.
An upper surface of diaphragm 513 receives the pressure of the refrigerant charged into first thermo-sensing tube 51b, pipe member 51c and first section 512a of chamber 512 such that a first force acts on diaphragm 513. The first force varies in response to changes in the temperature sensed by first thermo-sensing tube 51b. The lower surface of diaphragm 513 receives the pressure of the refrigerant at a position adjacent to first port 15a of external heat exchanger 15. Thus, an upwardly acting second force acts on diaphragm 513. The second force varies in response to changes in the pressure of the refrigerant in tube member 20 at the position adjacent to first port 15a of external heat exchanger 15. Therefore, a central portion of diaphragm 513 moves in accordance with the first force, the second force and the restoring force of coil spring 516, and rod 514 moves upwardly and downwardly in accordance with the movement of the central portion of diaphragm 513. As a result, the opening degree of valve element 514a at valve seat 518 varies in accordance with the movement of the central portion of diaphragm 513, so that first external pressure equalizing type expansion valve mechanism 51 has a P-T characteristic curve as illustrated in FIG. 3.
In FIG. 3, dashed line C.sub.1 indicates a saturated refrigerant vapor curve, and solid line C'.sub.2 indicates a static superheat curve with respect to both the first and second external pressure equalizing type expansion valve mechanisms 51, 52. SH.sub.2 indicates the degree of static superheat adjacent to first port 15a of external heat exchanger 15. As illustrated in FIG. 3, the static superheat curve C'.sub.2 is generally parallel to the saturated refrigerant vapor curve C.sub.1. Therefore, the static superheat SH.sub.2 is generally maintained at a predetermined constant value in any thermal condition of external heat exchanger 15.
With reference to FIG. 1 again, second valve device 502 is positioned between inlet port 12a of first internal heat exchanger 12 and fourth electromagnetic valve 164. One end of second pressure equalizing pipe 52a is hermetically inserted into tube member 20 at a position adjacent to outlet port 12b of first internal heat exchanger 12. Second thermo-sensing tube 52b is positioned adjacent to outlet port 12b of first internal heat exchanger 12, but downstream to second pressure equalizing pipe 52a. Second valve device 502 is similar in construction and operation to first valve device 501, so a detailed explanation is omitted.
Depending on the operational state of first through fourth electromagnetic valves 161-164, the automotive air conditioning system 100' performs in four operating modes: a cooling mode, a cooling-dehumidifying mode, a heating mode and a heating-dehumidifying mode.
First, in the cooling mode, first and second electromagnetic valves 161 and 162 are closed, and third and fourth electromagnetic valves 163 and 164 are open. Refrigerant flowing from compressor 11 flows to external heat exchanger 15 via third electromagnetic valve 163, and further flows to first internal heat exchanger 12 via second check valve 172, receiver 14, fourth electromagnetic valve 164 and second valve device 502. Then, the refrigerant returns to compressor 11 to complete the cycle. In the cooling mode, external heat exchanger 15 performs as a condenser while first internal heat exchanger 12 performs as an evaporator, which functions as a cooling source. Therefore, as the air flows through duct 110, the air is cooled by first internal heat exchanger 12. Furthermore, second damper 140 is positioned as indicated by the dashed line in FIG. 1 (i.e., entirely blocked), so that the air cooled by first internal heat exchanger 12 entirely passes second internal heat exchanger 13. The air cooled by first internal heat exchanger 12 is directed against the front windshield and/or the front lower portion of the automobile passenger compartment through the first and/or second auxiliary ducts 110a and/or 110b, respectively.
The electric signal S.sub.eva representing the sensed temperature T.sub.eva of the air at the position immediately downstream to first internal heat exchanger 12 is processed in the control unit. In response, the rotating speed of the drive shaft of compressor 11 is controlled. In other words, the displacement of compressor 11 is controlled so that the sensed temperature T.sub.eva approaches an aimed temperature Tao, which is expressed by equation (1). EQU Tao=Ks.multidot.Ts-Kr.multidot.Tr-Kam.multidot.Tam-Krad.multidot.Trad+C (1)
where;
Ks is the set temperature coefficient representative of impurities in the automobile passenger compartment air,
Ts is the set temperature of the automobile passenger compartment,
Kr is the sensed temperature coefficient in the automobile passenger compartment,
Tr is the sensed temperature in the automobile passenger compartment,
Kam is the ambient temperature coefficient outside the automobile,
Tam is the ambient temperature outside the automobile,
Krad is the coefficient of solar radiation,
Trad is the amount of solar radiation, and C is a constant.
In the cooling-dehumidifying mode, second electromagnetic valve 162 is closed, and the first, third and fourth electromagnetic valves 161, 163 and 164 are open. A portion of the refrigerant flowing from compressor 11 flows to external heat exchanger 15 via third electromagnetic valve 163, while the remainder flows to second internal heat exchanger 13 via first electromagnetic valve 161. The refrigerant flowing from external heat exchanger 15 and the refrigerant flowing from second internal heat exchanger 13 merge at receiver 14 via second and first check valves 172 and 171, respectively. The refrigerant then flows to first internal heat exchanger 12 via fourth electromagnetic valve 164 and second valve device 502, and returns to compressor 11. In the cooling-dehumidifying mode, external heat exchanger 15 and second internal heat exchanger 13 perform as condensers, while first internal heat exchanger 12 functions as an evaporator and cooling source. Second damper 140 is partially open. The position of the second damper 140 is determined by equation (2) below. EQU (Tao-T.sub.eva)/(T.sub.mix -T.sub.eva) (2)
As the air flows through duct 110, the air is cooled by first internal heat exchanger 12. The cooled air flowing out from first internal heat exchanger 12 is parted by second damper 140, so that a portion thereof passes through second internal heat exchanger 13 to be heated, while the remainder bypasses second internal heat exchanger 13. The air then merges at a position downstream to second internal heat exchanger 13. The merged air, which is cooled and dehumidified, is directed against the automobile front windshield and/or the front lower portion of the automobile passenger compartment through the first and/or second auxiliary ducts 110a and/or 110b, respectively.
The electric signal S.sub.eva representing the sensed temperature T.sub.eva of the air at the position immediately downstream to first internal heat exchanger 12 is processed in the control unit. In response, the rotating speed of the drive shaft of compressor 11 is controlled. In other words, the displacement of compressor 11 is controlled so that the sensed temperature T.sub.eva approaches the aimed temperature Tad.
In the heating mode, first and second electromagnetic valves 161 and 162 are open, and third and fourth electromagnetic valves 163 and 164 are closed. The refrigerant flowing from compressor 11 flows to second internal heat exchanger 13 via first electromagnetic valve 161, and further flows to external heat exchanger 15 via first check valve 171, receiver 14 and first valve device 501. The refrigerant flowing out of external heat exchanger 15 returns to compressor 11 via second electromagnetic valve 162. In the heating mode, external heat exchanger 15 performs as an evaporator, while second internal heat exchanger 13 performs as a condenser, which functions as a heating source. Furthermore, second damper 140 is positioned as indicated by the solid line in FIG. 1 (i.e., entirely opened), so that the air entirely passes through second internal heat exchanger 13. As the air flows through duct 110, the air passes through first internal heat exchanger 12 without a heat exchanging operation. The air flowing from first internal heat exchanger 12 entirely passes through second internal heat exchanger 13, and is heated thereby. The air heated by second internal heat exchanger 13 is directed against the automobile front windshield and/or the front lower portion of the automobile passenger compartment through the first and/or second auxiliary ducts 110a and/or 110b, respectively.
The electric signal S.sub.mix representing the sensed temperature T.sub.mix of the air at the position immediately downstream of second internal heat exchanger 13 is processed in the control unit. In response, the rotating speed of the drive shaft of compressor 11 is controlled. In other words, the displacement of compressor 11 is controlled so that the sensed temperature T.sub.mix approaches the aimed temperature Tao.
In the heating-dehumidifying mode, third electromagnetic valve 163 is closed, and the first, second and fourth electromagnetic valves 161, 162 and 164 are open. As a result, the refrigerant flowing from compressor 11 flows to second internal heat exchanger 13 via first electromagnetic valve 161, and further flows to receiver 14 via first check valve 171. The refrigerant flowing from receiver 14 divergently flows to external heat exchanger 15 via first valve device 501 and first internal heat exchanger 12 via fourth electromagnetic valve 164 and second valve device 502. The refrigerant flowing from first internal heat exchanger 12 merges with the refrigerant flowing from external heat exchanger 15 via second electromagnetic valve 162, and returns to compressor 11. In the heating-dehumidifying mode, external heat exchanger 15 and first internal heat exchanger 12 perform as evaporators, which function as cooling sources, while second internal heat exchanger 13 performs as a condenser, which functions as a heating source. Second damper 140 is maintained at the position indicated by the solid line in FIG. 1 (i.e., entirely opened).
As the air flows through duct 110, the air is cooled by first internal heat exchanger 12. The cooled air flowing from first internal heat exchanger 12 entirely passes through second internal heat exchanger 13, where it is heated. The air heated by second internal heat exchanger 13 is directed to the automobile front windshield and/or the front lower portion of the automobile passenger compartment through the first and/or second auxiliary ducts 110a and/or 110b, respectively.
The electric signal S.sub.mix representing the sensed temperature T.sub.mix of the air at the position immediately downstream of second internal heat exchanger 13 is processed in the control unit. In response, the rotating speed of the drive shaft of compressor 11 is controlled. In other words, the displacement of compressor 11 is controlled so that the sensed temperature T.sub.mix approaches the aimed temperature Tao.
When the heating mode is carried out and the temperature of the air outside the automobile is low, for example, -5.degree. C. .about.0.degree. C., the vaporizing pressure of the refrigerant in external heat exchanger 15 lowers, i.e, the temperature of the refrigerant in external heat exchanger 15 tends to lower. Since the condensing pressure of the refrigerant in second internal heat exchanger 13 is high, the pressure in the discharge chamber of compressor 11 tends to rise. These tendencies are aggravated when the degree of actual superheat SH.sub.2 a at a position adjacent to first port 15a of external heat exchanger 15 becomes excessively greater then the predetermined constant value of static superheat SH.sub.2. As a result, the operational efficiency of compressor 11 may decrease. A situation where the degree of actual superheat SH.sub.2 a becomes excessively greater than the degree of static superheat SH.sub.2 is described in detail below with reference to FIGS. 4 and 5.
As illustrated in FIG. 5, the amount of increase in the degree of superheat SH.sub.2 c at a position adjacent to first port 15a of external heat exchanger 15 is in direct proportion to the opening degree of valve element 514a (FIG. 2). On the other hand, when the temperature of the air outside the automobile is low, the opening degree of valve element 514a must be large. Therefore, when the temperature of the air outside the automobile is low, the increase in the degree of static superheat SHc at the position adjacent to first port 15a of external heat exchanger 15 becomes large. As a result, as illustrated in FIG. 4, a first acting point P.sub.1 of the first external pressure equalizing type expansion valve mechanisms 51 responsive to pressure sensed by first pressure equalizing pipe 51a is shifted from the static superheat curve C'.sub.2 along the axis of the abscissas toward the right (with respect to FIG. 4) by SHc.
In addition, a pressure drop .DELTA.P occurs between the position first pressure equalizing pipe 51a which is inserted in tube member 20 and first thermo-sensing tube 51b. As a result, as illustrated in FIG. 4, the pressure of the first external pressure equalizing type expansion valve mechanisms 51 sensed by first thermo-sensing tube 51b shifts to second acting point P.sub.2 along the axis of the ordinate. The second acting point P.sub.2 of the first external pressure equalizing type expansion valve mechanisms 51 is located on an actual superheat curve C'.sub.3.
As illustrated in FIG. 4, when the temperature of the air outside the automobile is low, the degree of actual superheat SH.sub.2 a becomes excessively greater than the predetermined constant value of the degree of static superheat SH.sub.2. In other words, as the temperature of the air outside the automobile lowers, the actual superheat SH.sub.2 a becomes greater.
Furthermore, the degree of actual superheat SH.sub.2 a becomes excessively larger than the predetermined constant value of static superheat SH.sub.2 when the temperature of the air outside the automobile is low. Therefore, the vaporizing pressure of the refrigerant in external heat exchanger 15 is further lowered, i.e., the temperature of the refrigerant in external heat exchanger 15 is further lowered. As a result, frost may form on the exterior surface of external heat exchanger 15 when the heating mode operation is carried out and the temperature of the air outside the automobile is low. The frost forming at the exterior surface of the external heat exchanger 15 may reduce the operational efficiency of the automotive air conditioning system 100'.
When the heating-dehumidifying mode is carried out and the temperature of the air outside the automobile is at an intermediate range, for example, 10.degree. C..about.20.degree. C., the dehumidifying operation at first internal heat exchanger 12 is not sufficiently carried out. In the heating-dehumidifying mode of operation, the refrigerant flowing from receiver 14 flows into external heat exchanger 15 and first internal heat exchanger 12. The ratio of heat exchanging capacity of external heat exchanger 15 to heat exchanging capacity of first internal heat exchanger 12 is designed to be about 3-10:1. Therefore, the greater part of the refrigerant flowing from receiver 14 flows into external heat exchanger 15. As a result, the heat exchanging operation at first internal heat exchanger 12 is not sufficiently carried out, so that the temperature of the air blowing from first internal heat exchanger 12 does not sufficiently rise. Therefore, the dehumidifying operation at the first internal heat exchanger 12 is not sufficiently carried out.